Fuel cell system

ABSTRACT

A fuel cell system includes a compressor provided in a cathode gas supply passage configured to feed the cathode gas under pressure to the fuel cell, a bypass passage configured to discharge the cathode gas fed under pressure by the compressor to a cathode gas discharge passage while bypassing the fuel cell, a bypass valve provided in the bypass passage and configured to adjust a flow rate of the cathode gas flowing in the bypass passage, a system stopping unit configured to stop the fuel cell system by performing a predetermined stop sequence process when a request to stop the fuel cell system is made, and a stop-time bypass valve control unit configured to control a valve body of the bypass valve to a predetermined initialization position in parallel with the sequence process during the stop sequence process.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent ApplicationNo. 2012-125759 filed with the Japan Patent Office on Jun. 1, 2012, allthe contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The prevent invention relates to a fuel cell system.

BACKGROUND

JP2008-293869A discloses a conventional fuel cell system in which acontroller is booted to initialize a valve after the elapse of apredetermined time after the operation of the fuel cell system isfinished.

SUMMARY

However, since the controller is booted to initialize the valve afterthe operation of the fuel cell system is finished in the aforementionedconventional fuel cell system, there is a possibility of producingunnecessary sounds after the system is finished.

On the other hand, if an initialization process is simply added tovarious stop processes (stop sequence process) to stop the fuel cellsystem, there is a possibility of making the stop sequence processitself longer.

The present invention was developed in view of such problems and aims tosuppress a stop sequence process from becoming longer while controllinga valve to an initialization position during the stop sequence processperformed when the operation of a fuel cell system is finished.

According to one aspect of the present invention, a fuel cell systemconfigured to generate power by supplying anode gas and cathode gas to afuel cell includes a compressor provided in a cathode gas supply passageand configured to feed the cathode gas under pressure to the fuel cell,a bypass passage configured to discharge the cathode gas fed underpressure by the compressor to a cathode gas discharge passage whilebypassing the fuel cell, a bypass valve provided in the bypass passageand configured to adjust a flow rate of the cathode gas flowing in thebypass passage, a system stopping unit configured to stop the fuel cellsystem by performing a predetermined stop sequence process when arequest to stop the fuel cell system is made, and a stop-time bypassvalve control unit for controlling a valve body of the bypass valve to apredetermined initialization position in parallel with the stop sequenceprocess during the stop sequence process.

An embodiment and advantages of the present invention are described indetail below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a fuel cell system according to oneembodiment of the present invention,

FIG. 2 is a flow chart showing control contents of a stop-time fullclosing process according to the embodiment of the present invention,

FIG. 3 is a flow chart showing control contents of a minimuminitialization process according to the embodiment of the presentinvention,

FIG. 4 is a time chart showing the operation of a stopping process ofthe fuel cell system according to the embodiment of the presentinvention, and

FIG. 5 is a time chart showing the operation of a starting process ofthe fuel cell system according to the embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

In a fuel cell, an electrolyte membrane is sandwiched by an anodeelectrode (fuel electrode) and a cathode electrode (oxidant electrode)and power is generated by supplying anode gas (fuel gas) containinghydrogen to the anode electrode and cathode gas (oxidant gas) containingoxygen to the cathode electrode. Electrode reactions which proceed inboth anode and cathode electrodes are as follows.Anode electrode: 2H₂→4H⁺+4e ⁻  (1)Cathode electrode: 4H⁺+4e ⁻+O₂→2H₂O  (2)

The fuel cell generates an electromotive force of about 1 volt by theseelectrode reactions (1) and (2).

In the case of using such a fuel cell as a power source for automotivevehicle, a fuel cell stack in which several hundreds of fuel cells arelaminated is used since required power is large. By configuring a fuelcell system for supplying the anode gas and the cathode gas to the fuelcell stack, power for driving a vehicle is taken out.

FIG. 1 is a schematic diagram of a fuel cell system 100 according to oneembodiment of the present invention.

The fuel cell system 100 includes a fuel cell stack 1, a cathode gassupplying/discharging device 2, an anode gas supplying/dischargingdevice 3, a power system 4 and a controller 5.

The fuel cell stack 1 is formed by laminating several hundreds of fuelcells and generates power necessary to drive a vehicle upon receivingthe supply of the anode gas and the cathode gas. The fuel cell stack 1includes an anode electrode side output terminal 11 and a cathodeelectrode side output terminal 12 as terminals for taking out power.

Further, the fuel cell stack 1 includes a current sensor 13 fordetecting a current taken out from the fuel cell stack 1 (hereinafter,referred to as a “stack output current”) and a voltage sensor 14 fordetecting an inter-terminal voltage between the anode electrode sideoutput terminal 11 and the cathode electrode side output terminal 12(hereinafter, referred to as a “stack output voltage”).

The cathode gas supplying/discharging device 2 is a device for supplyingthe cathode gas to the fuel cell stack 1 and discharging cathode off-gasdischarged from the fuel cell stack 1 to outside air. The cathode gassupplying/discharging device 2 includes a cathode gas supply passage 21,a filter 22, a cathode compressor 23, a cathode gas discharge passage24, a cathode pressure regulating valve 25, a bypass passage 26 and abypass valve 27.

The cathode gas supply passage 21 is a passage in which the cathode gasto be supplied to the fuel cell stack 1 flows. One end of the cathodegas supply passage 21 is connected to the filter 22 and the other end isconnected to a cathode gas inlet hole of the fuel cell stack 1.

The filter 22 removes foreign substances in the cathode gas to be takeninto the cathode gas supply passage 21.

The cathode compressor 23 is provided in the cathode gas supply passage21. The cathode compressor 23 takes air (outside air) as the cathode gasinto the cathode gas supply passage 21 via the filter 22 and supplies itto the fuel cell stack 1.

The cathode gas discharge passage 24 is a passage in which the cathodegas discharged from the fuel cell stack 1 flows. One end of the cathodegas discharge passage 24 is connected to a cathode gas outlet hole ofthe fuel cell stack 1, and the other end serves as an opening end.

The cathode pressure regulating valve 25 is provided in the cathode gasdischarge passage. The cathode pressure regulating valve 25 adjusts apressure of the cathode gas supplied to the fuel cell stack 1 to adesired pressure.

The bypass passage 26 is a passage for discharging a part of the cathodegas discharged from the cathode compressor 23 directly to the cathodegas discharge passage 24 while bypassing the fuel cell stack 1 to avoidhydrogen dilution and the surge of the cathode compressor 23. One end ofthe cathode gas bypass passage 26 is connected to a part of the cathodegas supply passage 21 downstream of the cathode compressor 23 and theother end is connected to a part of the cathode gas discharge passage 24downstream of the cathode pressure regulating valve 25.

The bypass valve 27 is provided in the bypass passage 26. The bypassvalve 27 is an on-off valve whose opening is adjusted in a stepwisemanner by a stepping motor 271 and configured such that the openingthereof is increased when the stepping motor 271 is rotated in a forwarddirection while being reduced when the stepping motor 271 is rotated ina reverse direction. By adjusting the opening of the bypass valve 27, aflow rate of the cathode gas bypassing the fuel cell stack 1 isadjusted.

The stepping motor 271 is a motor which is rotated by a predeterminedbasic angle every time a pulse signal is input, and a rotation speedthereof increases with an increase in the frequency of the input pulsesignal.

In the present embodiment, a number obtained by dividing a rotationangle of the stepping motor 271 necessary to fully close the bypassvalve 27 in a fully open state by the basic angle is called a stepnumber for the sake of convenience and the step number is defined to bezero when the bypass valve 27 is fully closed. The step number when thebypass valve 27 is fully open is called a fully open step number. In thepresent embodiment, the fully open step number is about 60.

The anode gas supplying/discharging device 3 is a device for supplyingthe anode gas to the fuel cell stack 1 and discharging anode off-gasdischarged from the fuel cell stack 1 to the cathode gas dischargepassage 24. The anode gas supplying/discharging device 3 includes ahigh-pressure tank 31, an anode gas supply passage 32, a cut-off valve33, an anode pressure regulating valve 34, an anode gas dischargepassage 35 and a purge valve 36.

The high-pressure tank 31 stores the anode gas to be supplied to thefuel cell stack 1 in a high-pressure state.

The anode gas supply passage 32 is a passage for supplying the anode gasdischarged from the high-pressure tank 31 to the fuel cell stack 1. Oneend of the anode gas supply passage 32 is connected to the high-pressuretank 31 and the other end is connected to an anode gas inlet hole of thefuel cell stack 1.

The cut-off valve 33 is provided in the anode gas supply passage 32. Byclosing the cut-off valve 33, the supply of the anode gas to the fuelcell stack 1 is stopped.

The anode pressure regulating valve 34 is provided in a part of theanode gas discharge passage 32 downstream of the cut-off valve 33. Theanode pressure regulating valve 34 adjusts a pressure of the anode gassupplied to the fuel cell stack 1 to a desired pressure.

The anode gas discharge passage 35 is a passage in which the anodeoff-gas discharged from the fuel cell stack 1 flows. One end of theanode gas discharge passage 35 is connected to an anode gas outlet holeof the fuel cell stack 1 and the other end is connected to the cathodegas discharge passage 24.

The purge valve 36 is provided in the anode gas discharge passage 35.The purge valve 36 adjusts a flow rate of the anode off-gas dischargedfrom the anode gas discharge passage 35 to the cathode gas dischargepassage 24.

The power system 4 includes a drive motor 41, an inverter 42, a powerdistribution device 43, a stack power breaker 44, a strong electricbattery 45, a strong electric power breaker 46, a voltage step-downconverter 47, a weak electric battery 48 and a weak electric powerbreaker 49.

The drive motor 41 is a three-phase alternating current synchronousmotor in which a permanent magnet is embedded in a rotor and a statorcoil is wound around a stator. The drive motor 41 has a function as amotor to be driven and rotated upon receiving the supply of power fromthe fuel cell stack 1 and the strong electric battery 45 and a functionas a generator for generating an electromotive force at opposite ends ofthe stator coil during the deceleration of the vehicle in which therotor is rotated by an external force.

The inverter 42 is composed of a plurality of semiconductor switchessuch as IGBTs (Insulated Gate Bipolar Transistors). The semiconductorswitches of the inverter 42 are controlled to be opened and closed bythe controller 5, whereby direct current power is converted intoalternating current power or alternating current power is converted intodirect current power. The inverter 42 converts composite direct currentpower of generated power of the fuel cell stack 1 and output power ofthe strong electric battery 45 into three-phase alternating currentpower and supplies it to the drive motor 41 when the drive motor 41 iscaused to function as a motor. On the other hand, the inverter 42converts regenerative power (three-phase alternating current power) ofthe drive motor 41 into direct current power and supplies it to thestrong electric battery 45 when the drive motor 41 is caused to functionas a generator.

The power distribution device 43 is a bilateral voltage converter forincreasing and decreasing an output voltage of the fuel cell stack 1. Inthe present embodiment, a DC/DC converter is used as the powerdistribution device 43. By controlling the stack output voltage by thepower distribution device 43, the generated power of the fuel cell stack1 (stack output current×stack output voltage) is controlled, the chargeand discharge of the strong electric battery 45 are controlled, andnecessary power is appropriately distributed and supplied to eachelectric component of a strong electric system such as the cathodecompressor 23 and the drive motor 41 and each electric component of aweak electric system such as the cathode pressure regulating valve 25,the bypass valve 27, the cut-off valve 33, the anode pressure regulatingvalve 34 and the purge valve 36.

The stack power breaker 44 is on-off controlled by the controller 5 toelectrically connect or disconnect the fuel cell stack 1 and the powerdistribution device 43.

The strong electric battery 45 is a secondary battery capable ofcharging and discharging. The strong electric battery 45 charges anexcess of the generated power of the fuel cell stack 1 and theregenerative power of the drive motor 41. The power charged into thestrong electric battery 45 is supplied to each electric component of thestrong electric system if necessary and supplied to each electriccomponent of the weak electric system via the voltage step-downconverter 47. In the present embodiment, a lithium ion battery having anoutput voltage of about 300 [V] is used as the strong electric battery45.

The strong electric battery 45 includes a temperature sensor 451 fordetecting a temperature of the strong electric battery 45 and an SOCsensor 45 for detecting a state of charge (SOC) of the strong electricbattery 45.

The strong electric power breaker 46 is on-off controlled by thecontroller 5 to electrically connect or disconnect the strong electricbattery 45 to or from the power distribution device 43 and the voltagestep-down converter 47. Further, the strong electric power breaker 46includes a current sensor 461 for detecting a current taken out from thestrong electric battery 45 (hereinafter, referred to as a “batteryoutput current”) and a voltage sensor 462 for detecting an outputvoltage of the strong electric battery 45 (hereinafter, referred to as a“battery output voltage”).

The voltage step-down converter 47 supplies power to each electriccomponent of the weak electric system while reducing an applied voltage.In the present embodiment, a DC-DC converter is used as the voltagestep-down converter 47.

The weak electric battery 48 is a secondary battery capable of chargingand discharging. The weak electric battery 48 stores power to besupplied to the electric components of the weak electric system during astarting process and a stopping process of the fuel cell system 100 inwhich power is not generated in the fuel cell stack 1. In the presentembodiment, a lead storage battery having an output voltage of about 14[V] is used as the weak electric battery 48.

The weak electric power breaker 49 is on-off controlled by thecontroller 5 to electrically connect or disconnect the voltage step-downconverter 47 and the weak electric battery 48 to or from each electriccomponent of the weak electric system.

The controller 5 is configured by a microcomputer including a centralprocessing unit (CPU), a read-only memory (ROM), a random access memory(RAM) and an input/output interface (I/O interface).

To the controller 5 are input signals necessary to control the fuel cellsystem 100 from various sensors such as a rotation speed sensor 51 fordetecting a rotation speed of the cathode compressor 23 and a startingswitch 62 for detecting a request to start/stop the fuel cell system 100besides the first current sensor 13, the second current sensor 461, thefirst voltage sensor 14, the second voltage sensor 462, the temperaturesensor 451 and the SOC sensor 452 described above.

The controller 5 stops the fuel cell system 100 by performing apredetermined stop sequence process when the starting switch 52 isturned off, i.e. when a request to stop the fuel cell system 100 ismade. On the other hand, the controller 5 starts the fuel cell system byperforming a predetermined start sequence process when the startingswitch 52 is turned on, i.e. when a request to start the fuel cellsystem 100 is made.

The stop sequence process is specifically a process for completelystopping the fuel cell system 100 by successively performing a dryingprocess for drying the fuel cell system 1, a stop VLC (Voltage LimitControl) process for reducing the stack output voltage to apredetermined limit voltage, a power generation stopping process forstopping power generation in the fuel cell stack 1, a strong electricstopping process for cutting off the supply of power to the strongelectric system and a weak electric stopping process for cutting off thesupply of power to the weak electric system after the starting switch 62is turned off.

The start sequence process is a process for starting power generation inthe fuel cell stack 1 by successively performing a weak electricstarting process for starting the supply of power to the weak electricsystem, a strong electric starting process for starting the supply ofpower to the strong electric system and a stack starting process forperforming startup preparation of the fuel cell stack 1 after thestarting switch 62 is turned on.

In the present embodiment, a valve body of the bypass valve 27 iscontrolled to a fully closed position during these stop sequence processand start sequence process. The reason for that is described below.

As described above, the bypass valve 27 is an on-off valve whose openingis adjusted in a stepwise manner by the stepping motor 271.

Since the stepping motor 271 includes no means for directly detecting anactual rotational position, the opening of the bypass valve 27, i.e. avalve body position of the bypass valve 27 is not known immediatelyafter the start of the fuel cell system 100. Thus, an initializationprocess for grasping the position of the valve body by rotating thestepping motor 271 in the reverse direction to press the valve body ofthe bypass valve 27 against a valve seat and fully close the opening ofthe bypass valve 27 is necessary before the start of power generation inthe fuel cell stack 1 when the fuel cell system 100 is started.

Once the initialization process is performed, the step number can becalculated according to the number of pulse signals input to thestepping motor 271 until the fuel cell system 100 is stopped next. Thus,the opening of the bypass valve 27 can be grasped.

Here, since the valve body position of the bypass valve 27 is not knownbefore the initialization process is performed, the stepping motor 271needs to be rotated in the reverse direction at least by the fully openstep number to reliably pressing the bypass valve 27 against the valveseat and fully close the valve body 27.

Then, even after the valve body reaches the valve seat during theexecution of the initialization process, the stepping motor 271 might bepossibly rotated in the reverse direction. If the stepping motor 271 isrotated in the reverse direction even after the valve body reaches thevalve seat, the wear of parts increases and sound vibration performanceis deteriorated since the valve body is kept pressed against the valveseat. Further, step-out may occur by the valve seat being bounced backby the valve body.

Thus, during the initialization process, the wear of parts, thedeterioration of sound vibration performance and the occurrence ofstep-out as just described need to be suppressed by setting the rotationspeed of the stepping motor 271 slower than in normal time. Note thatnormal time mentioned here means a time during which power is generatedin the fuel cell stack 1 and the fuel cell system 100 is operated withthat generated power.

The bypass valve 27 controls the flow rate of the cathode gas suppliedto the fuel cell system 1 by controlling the flow rate of the cathodegas flowing in the bypass passage 26. Thus, the initialization processof the bypass valve 27 needs to be performed before power generation isstarted in the fuel cell stack 1. However, it takes time if the steppingmotor 271 is rotated in the reverse direction by the fully open stepnumber at the speed slower than in normal time, and a time until powergeneration is started in the fuel cell stack 1 after the start of thefuel cell system 100 becomes longer. Then, a time until warm-up iscompleted after the start of the fuel cell system 100 becomes longer asa result of that, wherefore a time until travel is allowed after thestart becomes longer to deteriorate merchantability.

Accordingly, in the present embodiment, a stop-time full closing processfor controlling the stepping motor 271 to fully close the bypass valve27 is performed in parallel with the stop sequence process when it is nolonger necessary to supply the cathode gas to the fuel cell stack 1 andit becomes unnecessary to control the bypass valve 27 during theexecution of the stop sequence process.

When it becomes possible to supply power to the stepping motor 271 ofthe bypass valve 27 during the execution of the start sequence processwhen the fuel cell system 100 is started next time, the minimuminitialization process for initializing the bypass valve 27 by rotatingthe stepping motor 271 in the reverse direction by a predeterminedinitialization step number smaller than the fully open step number isperformed in parallel with the start sequence process. In the presentembodiment, the initialization step number is set to be about 8.

By fully closing the bypass valve 27 in advance in this way when thefuel cell system 100 is stopped, it is possible to initialize the bypassvalve 27 by the initialization step number smaller than the fully openstep number when the fuel cell system is started.

Thus, a time necessary to initialize the bypass valve 27 can beshortened and the time until power generation in the fuel cell stack 1is started after the start of the fuel cell system 100 can be shortened.

Further, to finish the stop-time full closing process during the stopsequence process, the stop-time full closing process is performed inparallel with the stop sequence process when it is no longer necessaryto supply the cathode gas to the fuel cell stack 1 and it becomesunnecessary to control the bypass valve 27. Thus, the stop-time fullyclosing process is not added as one process of the stop sequenceprocess. Therefore, an execution time of the stop sequence process isnot extended.

Control contents of the stop-time full closing process performed duringthe stop sequence process of this fuel cell system 100 and the minimuminitialization process performed during the start sequence process ofthe fuel cell system 100 are described below.

FIG. 2 is a flow chart showing the control contents of the stop-timefull closing process according to the present embodiment.

In Step S1, the controller 5 determines whether or not such anabnormality that the stop-time full closing process cannot be performedduring the operation of the fuel cell system 100 has occurred. Thecontroller 5 performs a processing of Step S2 if the abnormality hasoccurred. On the other hand, a processing of Step S3 is performed unlessthe abnormality has occurred.

In Step S2, the controller 5 stops the execution of the stop-time fullclosing process during the stop sequence process.

In Step S3, the controller 5 determines whether or not the dryingprocess has been finished. The drying process is a process fordischarging moisture in the fuel cell stack 1 to the outside of thesystem by driving the cathode compressor 23 for a predetermined timewith the generated power of the fuel cell stack 1 in preparation for thenext start. In this way, the deterioration of startability caused byfrozen moisture in the fuel cell stack 1 is prevented. The controller 5finishes the process this time unless drying has been finished whileperforming a processing of Step S4 if the drying process has beenfinished.

In Step S4, the controller 5 stops the cathode compressor 23 by settinga current carrying amount to the cathode compressor 23 at zero.

In Step S5, the controller 5 determines whether or not a rotation speedN of the cathode compressor 23 has dropped to or below a stopdetermining rotation speed Ns. The controller 5 finishes the processthis time if the rotation speed N of the cathode compressor 23 is higherthan the stop determining rotation speed Ns. On the other hand, aprocessing of Step S6 is performed unless the rotation speed N of thecathode compressor 23 is higher than the stop determining rotation speedNs.

In Step S6, the controller 5 determines whether or not the step numberof the stepping motor 271 of the bypass valve 27 is not larger than theinitialization step number. The controller 5 performs a processing ofStep S7 if the step number of the stepping motor 271 is larger than theinitialization step number while performing a processing of Step S8unless it is larger than the initialization step number.

In Step S7, the controller 5 rotates the stepping motor 271 in thereverse direction at a rotation speed in normal time so that the stepnumber reaches the initialization step number.

In Step S8, the controller 5 rotates the stepping motor 271 in thereverse direction at a rotation speed slower than in normal time so thatthe step number becomes zero.

FIG. 3 is a flow chart showing the control contents of the minimuminitialization process according to the present embodiment.

In Step S11, the controller 5 determines whether or not the stop-timefull closing process has been performed during the stop sequenceprocess. The controller 5 performs a processing of Step S12 if thestop-time full closing process has been performed during the stopsequence process. On the other hand, a processing of Step S13 isperformed unless the stop-time full closing process has been performedduring the stop sequence process.

In Step S12, the controller 5 rotates the stepping motor 271 in thereverse direction at a rotation speed slower than in normal time by theinitialization step number. The stepping motor 271 is rotated in thereverse direction by the initialization step number in this way when thestop-time full closing process has been performed during the stopsequence process because the valve body position of the bypass valve 27can be predicted to be near the fully closed position even when it isdeviated from the fully closed position before the next start if thestop-time full closing process has been performed during the stopsequence process, and the valve body can be sufficiently pressed againstthe valve seat only by rotation in the reverse direction by theinitialization step number smaller than the fully open step number.

In Step S13, the controller 5 rotates the stepping motor 271 in thereverse direction at a rotation speed slower than in normal time by thefully open step number. The stepping motor 271 is rotated in the reversedirection by the fully open step number in this way when the stop-timefull closing process has not been performed during the stop sequenceprocess because the valve body position of the bypass valve 27 is notknown.

FIG. 4 is a time chart showing the operation of the stop sequenceprocess according to the present embodiment.

When the starting switch 62 is turned off at time t1, the drying processis performed. During the drying process, the anode gas and the cathodegas are supplied to the fuel cell stack 1 and the cathode compressor 23is driven by the generated power of the fuel cell stack 1.

When the drying process is finished at time t2, energization to thecathode compressor 23 is stopped to perform the stop VLC process.

When the rotation speed of the cathode compressor 23 drops to the stopdetermining rotation speed Ns at time t3, the stop-time full closingprocess is performed and the bypass valve 27 is controlled to be fullyclosed. The stop-time full closing process is performed in this wayafter the cathode compressor 23 is stopped because the opening of thebypass valve 27 needs to be controlled while the cathode compressor 23is driven.

Further, at time t3, the stop VLC process is simultaneously performed.The stop VLC process is a process for consuming the cathode gas in thefuel cell stack 1 by supplying only the anode gas and generating powerafter the supply of the cathode gas is stopped and reducing the stackoutput voltage to a limit voltage. In this way, it is possible toprevent the deterioration of catalysts of the fuel cells caused by thestop of the fuel cell system 100 with the stack output voltage kepthigh.

When the stack output voltage drops to the limit voltage at time t4, thepower generation stopping process is performed and the cut-off valve 33is fully closed after the anode pressure regulating valve 34 is fullyclosed. Finally, the stack power breaker 44 is cut off.

When the power generation stopping process is finished at time t5, thestrong electric stopping process is performed to prepare for cutting offthe strong electric power breaker 46.

When the strong electric stopping process is finished and the strongelectric power breaker 46 is cut off at time t6, the weak electricstopping process is performed to prepare for cutting off the weakelectric power breaker 49.

A period from time t3 to time t6 until the weak electric stoppingprocess is started after the drive of the cathode compressor 23 isstopped is a period during which the stop-time full closing process ofthe bypass valve 27 can be performed.

When the weak electric stopping process is finished at time t7, the weakelectric power breaker 49 is cut off. In this way, the fuel cell system100 is completely stopped.

FIG. 5 is a time chart showing the operation of the start sequenceprocess according to the present embodiment.

When the starting switch 62 is turned on at time t11, the weak electricpower breaker 49 is connected and the weak electric starting process isstarted. In the weak electric starting process, self-diagnosis of thecontroller 5, diagnosis on seizure of the weak electric power breaker 49and the like are made in the weak electric starting process.

When the weak electric starting process is finished at time t12, thestrong electric power breaker 46 is connected, the strong electricstopping process is started and the minimum initialization process ofthe bypass valve 27 is started. The minimum initialization process isstarted simultaneously with the end of the weak electric startingprocess in this way because power of the weak electric battery can besupplied to the stepping motor 271 of the bypass valve 27 and theminimum initialization process can be performed when the weak electricstarting process is finished.

It should be noted that diagnosis on seizure of the strong electricpower breaker 46, judgment as to whether or not the battery outputvoltage has risen to a predetermined voltage or higher and the like canbe made in the strong electric stopping process.

When the strong electric stopping process is finished at time t13, thestack power breaker 44 is connected after the stack starting process isperformed and the cut-off valve 33 is opened. Thereafter, the anodepressure regulating valve 34 is opened and the cathode compressor 23 isdriven to start power generation of the fuel cell stack 1.

The minimum initialization process of the bypass valve 27 can beperformed during a period from time t12 to time t14 until the cathodecompressor 23 is driven to supply the cathode gas to the fuel cell stack1 after the weak electric starting process is finished.

According to the present embodiment described above, the stop-time fullclosing process is performed in parallel with the stop sequence processso that the stop-time full closing process is finished during the stopsequence process when it is no longer necessary to supply the cathodegas to the fuel cell stack 1 and it becomes unnecessary to control thebypass valve 27.

Since the stop-time full closing process needs not to be added as oneprocess of the stop sequence process as just described, it can besuppressed that the execution time of the stop sequence process becomeslonger.

Further, by fully closing the bypass valve 27 in advance when the fuelcell system 100 is stopped, the bypass valve 27 can be initialized bythe initialization step number smaller than the fully open step numberwhen the fuel cell system is started. Thus, a time until powergeneration is started in the fuel cell stack 1 after the start of thefuel cell system 100 can be shortened, wherefore a time until warm-up iscompleted after the start of the fuel cell system 100 can be shortened.Hence, a time until travel is allowed after the start can be shortened.

Further, since the stop sequence process is performed before the fuelcell system is completely stopped, the opening of the bypass valve canbe grasped. Thus, the valve body of the bypass valve needs not bepressed against the valve seat in controlling the bypass valve to thefully closed position unlike in the initialization process. Thus, thewear of parts can be suppressed and the deterioration of sound vibrationperformance can also be suppressed. Further, step-out caused by bounceback can be suppressed.

Further, according to the present embodiment, the rotation speed of thestepping motor 271 is set slower than in normal time when the stepnumber of the stepping motor 271 of the bypass valve 27 has dropped tothe initialization step number or smaller during the stop-time fullclosing process.

In this way, step-out caused by bounce back when the valve body of thebypass valve 27 reaches the valve seat can be reliably suppressed.Further, the wear of parts and the generation of sound vibration can befurther suppressed.

Further, according to the present embodiment, the stop-time full closingprocess is performed simultaneously with the stop VLC process.

Since the bypass valve 27 is controlled to be fully closed during thestop VLC process in this way, the reverse flow of mixed gas of thecathode off-gas discharged into the cathode gas discharge passage 24during the step VLC process and the anode off-gas into the bypasspassage 26 can be suppressed.

It should be noted that since the stepping motor 271 of the bypass valve27 is a part of the weak electric system, the stop-time full closingprocess can be performed until the weak electric stopping process isstarted after the cathode compressor 23 is stopped. However, by beingperformed simultaneously with the stop VLC process first performed afterthe stop of the cathode compressor 23, the stop-time full closingprocess can be finished at an early stage of the stop sequence processand reliably finished during the stop sequence process.

Although the embodiment of the present invention has been describedabove, the above embodiment is merely an illustration of one applicationexample of the present invention and not of the nature to limit thetechnical scope of the present invention to the specific configurationof the above embodiment.

For example, although the initialization position of the bypass valve 27is the fully closed position in the above embodiment, it may be a fullyopen position.

Further, although the stop-time full closing process is performedsimultaneously with the stop VLC process in the above embodiment, it maybe started during the stop VLC process or during the subsequent powergeneration stopping process or strong electric stopping process as longas the stop-time full closing process is finished before the weakelectric stopping process is started.

The invention claimed is:
 1. A fuel cell system configured to generate power by supplying anode gas and cathode gas to a fuel cell, comprising: a compressor provided in a cathode gas supply passage and configured to feed the cathode gas under pressure to the fuel cell; a bypass passage configured to discharge a portion of the cathode gas fed under pressure by the compressor to a cathode gas discharge passage while bypassing the fuel cell; a cathode gas discharge passage in which the cathode gas discharged from the fuel cell flows, the cathode gas discharge passage being connected to the bypass passage; an anode gas discharge passage in which an anode gas discharged from the fuel cell flows, the anode gas discharge passage being connected to the cathode gas discharge passage; a bypass valve provided in the bypass passage and configured to adjust a flow rate of the cathode gas flowing in the bypass passage; a system stopping unit configured to stop the fuel cell system by performing a predetermined stop sequence process when a request to stop the fuel cell system is made; a stop-time bypass valve control unit configured to perform a stop-time full closing process, the stop-time full closing process being a process of controlling a valve body of the bypass valve to a fully closed initialization position in a voltage limiting process during the stop sequence process for reducing a voltage of the fuel cell after the compressor is stopped; and a stepping motor configured to drive the bypass valve; a determination unit configured to determine whether or not the stop-time full closing process has been performed by the stop-time bypass valve control unit; and a minimum initialization unit configured to rotate the stepping motor after the stop sequence process, wherein: the stop-time bypass valve control unit is further configured to: start to control the bypass valve during the voltage limiting process; and control a rotation speed of the stepping motor so that the rotation speed when a step number of the stepping motor is lower than a predetermined step number becomes slower than the rotation speed when the step number of the stepping motor is higher than the predetermined step number; and the minimum initialization unit is further configured to: when the determination unit determines that the stop-time full closing process has been performed, rotate the stepping motor by an initialization step number smaller than a fully open step number; and when the determination unit determines that the stop-time full closing process has not been performed, rotate the stepping motor by the fully open step number.
 2. The fuel cell system according to claim 1, further comprising: a strong electric battery; and a weak electric battery having a lower electromotive force than the strong electric battery, wherein the stop sequence process includes: a power generation stopping process for stopping power generation in the fuel cell after the voltage limiting process is finished, the power generation stopping process including: a strong electric stopping process for stopping the supply of power from the strong electric battery after the voltage limiting process is finished, and a weak electric stopping process for stopping the supply of power from the weak electric battery after the strong electric stopping process is finished; and the stop-time bypass valve control unit starts the control of the bypass valve so as to be able to control the bypass valve to the initialization position before the weak electric stopping process is started.
 3. The fuel cell system according to claim 1, wherein: the stop-time bypass valve control unit sets a moving speed of the bypass valve slower than a speed of moving a valve body of the bypass valve to a predetermined position when a valve body position of the bypass valve reaches the predetermined position near the initialization position.
 4. The fuel cell system according to claim 1, wherein: the stop-time bypass valve control unit determines that the compressor has stopped when a rotation speed of the compressor drops to or below a predetermined speed.
 5. The fuel cell system according to claim 1, wherein, during control of the bypass valve during the voltage limiting process, the stop-time bypass valve control unit is further configured to: determine a step number of the stepping motor of the bypass valve; determine an initialization step number; and if the step number is larger than the initialization step number, rotate the stepping motor at a normal rotation speed to reach the initialization step number; and if the step number is smaller than the initialization step number, rotate the stepping motor at a slower than normal speed until the step number is zero.
 6. The fuel cell system according to claim 1, wherein the stop-time bypass valve control unit is further configured to: on condition that the step number is larger than the predetermined step number, rotate the stepping motor at a first rotation speed; and on condition that the step number is smaller than the initialization step number, rotate the stepping motor at a second rotation speed that is slower than the first rotation speed. 